In the foundation drilling industry, it is desired to drill relatively large diameter shafts (on the order of 36 inches to 48 inches and up) in the earth, and these shafts are typically filled with reinforced concrete to form foundation piles for buildings, bridges, etc. Often a complete shaft is drilled, such as by auguring. Alternatively, in the so-called drill shaft construction technique, a large diameter, hollow core barrel is rotated so that cutters on its lower edge cut an annular kerf in the ground, which is typically rock or rocky ground. Once this kerf is drilled to the desired depth by the core barrel's cutting face, the rock core within the kerf may be broken up and augured out, or broken off and removed to permit the shaft to be filled with reinforced concrete for forming a pile. Alternatively, the core may be left in place, with the pile being formed by filling the annular kerf with cementitious material, steel casement, or other suitable means for forming the outermost portion of the pile. An example of the latter technique is disclosed in my U.S. application Ser. No. 08/780,097 filed Dec. 24, 1996, now U.S. Pat. No. 5,823,276, the contents of which are incorporated herein by reference.
The use of sharp cutters to score the rock and form removable cuttings is but one way to drill shafts. In the prior art, there are also rotating, double-walled core barrels that have roller bits as the cutting surface. These roller bits are typically welded to the bottom of the barrel. As the core barrel rotates and the cutters scrape cuttings from the bottom of the kerf, pressurized air is circulated down between the double walls via a swivel through the rotary, thereby flushing cuttings up past the outer diameter of the core barrel and out of the kerf. The foregoing cutting techniques generally require extreme downward pressure on the core barrel.
For applications which only require smaller-diameter shafts, such as oil and gas drilling, it is known to use pneumatic, percussive-type downhole drills, which permit significant reductions in the amount of downward pressure that must be applied to the drilling apparatus. These small downhole "hammer" drills typically employ a drill bit with a circular cutting face having numerous protruding tungsten carbide buttons. A rotary head or kelly-bar drive causes the drill string to rotate in the shaft, and drilling pipes conduct compressed air to a piston (i.e., the hammer) near the end of the drill string, generating percussive blows of the cutting face of the drill bit to the earth at the bottom of the shaft. These percussive blows place the rock in compression, and the retreating drill bit places the rock in tension. This cyclic action, which may occur several hundred times per minute, breaks up the rock, which is then removed by a drilling fluid (often, simply air) which is circulated down into the shaft under pressure. Rotation of the drill string brings the drill bit into contact with fresh unbroken rock during successive percussion cycles.
Single downhole drills of the type described are typically from a few inches up to about 34 inches in diameter. Greater diameters are impractical due to the excessive cost of larger-diameter drill bits and large downhole hammers. To achieve larger-diameter shafts, it is known to use cluster drills comprising a plurality of hammer drills in a gang construction, as described in U.S. Pat. No. 4,729,439 to Kurt. In gang drills of this type, several hammer drills are arranged within a casing in a ring around a central hammer drill which is concentric with the casing and thus the shaft to be drilled. The cutting faces of the drill bits must be sufficiently large to cut swaths which completely cover the bottom of the shaft. For relatively large diameter shafts, e.g., 36 inches and greater, the number and size of hammer drills required makes their use impractical because air and fuel consumption tends to be quite high. In addition, noise levels for gang drills of this size (and large-diameter single downhole drills) may be intolerable, particularly if drilling is to occur near populated areas. Also, gang drills of this size suffer from disadvantages such as excessive weight and cost, and limited ability to be manhandled.